(a) Field of the Invention
The present invention relates to liquid crystal displays (LCDs) having storage capacitors, manufacturing methods and a driving method thereof.
(b) Description of the Related Art
In general, a thin film transistor liquid crystal display (TFT-LCD) includes data lines transmitting display signals, gate lines transmitting scan signals, thin film transistors (TFTs) as switching devices, liquid crystal capacitors and storage capacitors. The TFT-LCDs are classified into two modes according to structures of the storage capacitors. One mode has a separate storage line connected to the storage electrodes and the other mode connects the storage capacitor to a gate line.
The principle of driving the LCD in the former mode and a conventional structure of an LCD will be described.
FIG. 1 is an equivalent circuit diagram of a conventional LCD.
A plurality of gate lines G1 and G2 and a plurality of data lines D1, D2 and D3 extend respectively in the horizontal direction and in the vertical direction. The gate lines G1 and G2 and the data lines D1, D2 and D3 intersect each other to define a plurality of pixels. A storage wire COM1 and COM2 passes through the pixels and a TFT is formed in each pixel. A gate electrode (g) of the TFT is connected to the gate line G1 or G2, and a source and a drain electrodes (s and d) of the TFT are respectively connected to the data line and a liquid crystal capacitor (LC). The drain electrode (d) is connected to the storage wire COM 1 or COM 2 to form a storage capacitor (STG).
If a gate-on voltage is applied to the gate electrode (g) of the TFT through the gate line G1, a display signal voltage from the data line is transmitted to the pixels via the TFT and charged in the liquid crystal capacitor (LC) and the storage capacitor (STG). The charged voltage is maintained until the next gate-on voltage in the next frame is applied in the pixel. Generally, when the gate voltage is changed from "on" level to "off" level, the pixel voltage slightly drops. The storage capacitor reduces the voltage-drop.
The TFTs have amorphous silicon layers or polycrystalline silicon layers as active layers and are classified into a top gate mode and a bottom gate mode according to the relative location of the gate electrode and the active layer. Most of the polycrystalline silicon TFT-LCDs use the top gate mode.
A storage capacitor of the conventional polycrystalline silicon TFT-LCD includes a doped storage region in the silicon layer, a storage electrode overlapping the storage region and a gate insulating film interposed therebetween. Moreover, another storage capacitor is formed of the storage electrode, a pixel electrode overlapping the storage electrode and a dielectric including an interlayer insulating film and a passivation film interposed between the pixel electrode and the storage electrode. However, the capacitance between the pixel electrode and the storage electrode is relatively small and negligible, since the interlayer insulating film and the passivation film, which is respectively 5,000 .ANG. thick, are much thicker than the gate insulating film, which is 500 .ANG..about.3,000 .ANG. thick.
In this conventional structure, it is required to add an ion implanting step to make a storage region. In other words, additional steps of depositing photoresist film, patterning the photoresist to make openings by using a mask, injecting ions into the silicon layer through the openings and annealing the injected ions are required.